What is high-speed machining (HSM)? Adam Schaut and his colleagues at Boeing, St. Louis, Phantom Works division, prefer the definition given by Dr. Scott Smith of the University of North Carolina-Charlotte: "HSM occurs when the tooth pass frequency (RPM / 60 * number of teeth) approaches a substantial fraction of the dominant natural frequency of the system." This definition states that HSM involves more than just high spindle speeds. Itoccurs when the spindle speed is optimized for a particular tool in a particular machine.

Chatter is a primary concern in HSM. Chatter is more problematic at high speeds, due to the loss of process damping (in other words, the wavelengths of work-piece vibrations increase due to the high cutting velocities, which results in the loss of friction on the clearance face of the tool). Chatter is a self-excited vibration between the cutting tool and the work-piece. It is the result of a cutting tooth cutting over a portion of the work-piece that was machined by a previous cutter tooth, which changes the effective chip thickness and resulting tool forces. Chatter creates large cutting forces that may accelerate tool wear, often causing catastrophic tool failure. Chatter results in surface finishes that may be unacceptable, requiring part rework or, worse yet, complete part rejection.

The key to high-speed machining and the elimination of chatter is in the choice of the spindle speed. The optimum spindle speed is chosen by matching the chatter frequency with the tooth passing frequency of the cutter. The chatter frequency is directly related to the most dominant natural frequency of the machine tool system. For example, if the chatter frequency of a particular process is found to be 2000 Hz (cycles/second), then the goal is to match this chatter frequency with the tooth pass frequency of the tool. Given a two-tooth cutter, the optimum spindle speed would be 60,000 RPM (2000 cycles/second multiplied by 60 seconds/minute divided by 2 teeth equates to 60,000 revolutions/minute). If your machine is not capable of running at 60,000 RPM, then choose an integer fraction of this spindle speed. By matching the spindle speeds of the chatter frequency, regions of higher stability—known as stability zones—are achieved. Once a stability zone or optimum cutting speed is found for a particular tool and machine, then the depth-of-cut can be increased until a new chatter limit is encountered.

The future of high-speed machining is very bright. It is hopeful that all part manufacturers—from giants like Boeing to the small job shops—will be implementing good HSM techniques. This improved understanding throughout the industry will help drive the machine tool builders, such as Makino, and tool manufactures to improve their products and produce faster, more reliable, more accurate, and longer-lasting machines and tools. With these improvements come bigger, more accurate, monolithic machined parts, and ultimately faster, better, and less costly aircraft.

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